Article pubs.acs.org/OPRD
Selective Hydrogenation of Vegetable Oils over SiliaCat Pd(0) Valerica Pandarus,† Genevieve Gingras,† François Béland,*,† Rosaria Ciriminna,‡ and Mario Pagliaro*,‡ †
SiliCycle Inc., 2500 Parc-Technologique Blvd, Quebec City, Quebec G1P 4S6, Canada Istituto per lo Studio dei Materiali Nanostrutturati, CNR, via U. La Malfa 153, 90146 Palermo, Italy
‡
ABSTRACT: Nanostructured immobilized-Pd catalyst SiliaCat Pd(0) selectively mediates at room temperature the selective and complete hydrogenation of a wide variety of vegetable oils under hydrogen balloon conditions over 0.1 mol % (based on metal content) entrapped catalyst. No cis/trans isomerisation takes place, whereas the catalyst is truly recyclable with low leached amounts of valued palladium, thereby providing the fat and oleochemicals industry with a suitable replacement for Ni-based catalysts. mediator for the selective hydrogenation of nitroarenes.10 Now we report the discovery that SiliaCat Pd(0) is a chemoselective and efficient heterogeneous catalysts for the leach-proof hydrogenation of a wide variety of vegetable oils in a simple hydrogen balloon at room temperature. An ultralow amount of catalyst (0.1 mol %) is enough to promote full hydrogenation of FA, FAME, and vegetable oils with no isomerization and with negligible leaching of valued Pd. The catalyst is fully reusable, opening the route to replacement of Ni catalyst in the fat and oleochemicals industry.
1. INTRODUCTION Hardening of unsaturated fatty acids (FA) or fatty acid methyl esters (FAME) through catalytic hydrogenation is a fundamental industrial reaction carried out worldwide on very large scale since the early 1900s to manufacture basic oleochemicals, such as stabilizers and surfactants, and edible fats, such as margarine.1 Average annual world oil production in the years 1996−2000 amounted to 105 × 106 t and is expected to increase to 185 × 106 t in the years 2016−2020, with about 80% of the global oil and fat production being made of vegetable oils.2 From the beginning, the hydrogenation of FA or FAME has been heterogeneously catalyzed with Ni in suspension (today over Ni finely divided and supported on alumina or kieselguhr),3 mostly in a batch process in a reactor under relatively high H2 pressure (15−20 bar) and elevated temperature (180−230 °C) followed by the removal of the catalyst by time-consuming filtration with frequent Ni-soaps formation. Under these conditions nickel can indeed react with components of the fat to form fat-soluble salts which may contaminate the product,4 while a significant degree of cis/trans isomerization leads to formation of unhealthy trans fatty acid chains. To improve the hardening process and eliminate these disadvantages either fixed bed methods using Ni-based catalysts have been developed5 or replacement of nickel with palladium has been investigated (since the early 1960s).6 Research nowadays continues in the context of biomass valorization through hydrogenation.7 Yet, results of pure margarines production over Pd catalysts thus far have been uneconomical, as the amount of costly palladium employed needs to be significantly reduced and the catalyst recycled.8 Furthermore, because the specification limit for the residual Pd in food is 10fold lower than that of micronutrient Ni (100 vs 1,000 μg/day oral tolerable daily intake upper limit, respectively),9 the amount of Pd catalyst leached should be minimized; otherwise, Ni will continue to be used even though Ni in general shows lower activity than Pd. In this context, we recently reported that a newly developed series of commercial sol−gel entrapped Pd catalyst trade named SiliaCat Pd(0) made of ultrasmall Pd nanoparticles (4−6 nm large) encapsulated within an organosilica matrix is a good © 2012 American Chemical Society
2. EXPERIMENTAL SECTION The catalyst preparation has been described in detail elsewhere.11 Herein a 0.1−1 mol % SiliaCat Pd(0) sol−gel entrapped catalyst amount was employed (0.05 mmol g−1 Pd load from CAMECA SX100 equipped with an EPMA microanalyzer). Reactions were performed on a 3, 4, 5, or 6 mmol substrate scale dissolved in 20 mL of HPLC grade solvent for 0.15 M, 0.2 M, 0.25 M, or 0.3 M concentration, or on a 14.5 mmol substrate scale dissolved in 30 mL of THF or a THF/MeOH mixture (5:1 by volume ratio) for 0.5 M concentration. The mixture was degassed two times, replacing each time the vacuum by hydrogen. The reaction mixture connected to a hydrogen balloon was vigorously stirred at room temperature until it showed maximum conversion. The isolated yields are not given because during isolation of the product we lose a small amount of the product. THF (50 mL) was then added and the catalyst filtered and rinsed with extra THF, or the reaction mixture was heated at 40 °C and the catalyst filtered and rinsed with THF. The filtrate was concentrated to give a crude product that was each time analyzed by ICP-OES for leaching of Pd and Si. In the reusability test the hydrogenation was carried out in a hydrogen balloon at room temperature in the presence of the same sol−gel catalyst. A two-necked flask connected to a balloon of hydrogen was charged with the substrate, the solvent, and SiliaCat Pd(0) catalyst in 0.1 mol % amount. The mixture was degassed two times, replacing each time the Received: May 7, 2012 Published: June 15, 2012 1307
dx.doi.org/10.1021/op300115r | Org. Process Res. Dev. 2012, 16, 1307−1311
Organic Process Research & Development
Article
Hydrogenation of linolenic acid (cis C18:3, 3 (Figure 3)) involved consecutive saturation to cis C18:1 in 100% conversion after 2 h and subsequent saturation of cis C18:1 to cis C18:0 after 3 more hours (entry 3). Also the hydrogenation of eicosapentaenoic acid ethyl esther (EPA, cis C20:5, 4 (Figure 4)) resulted in complete saturation to cis C20:0 with 100% conversion after 3 h; similarly to the hydrogenation of erucic acid (cis C22:1, 5 (Figure 5)), affording complete saturation to cis C22:0 after 3 h. Again, isomerization reactions of cis to trans isomers were not observed. To explore the scope of the method, different vegetable oils were then hydrogenated under the same optimized reaction conditions of Table 1 in THF/MeOH (0.5 M) over 0.1 mol % SiliaCat Pd(0). The results in Table 4 show that, in general, complete substrate conversion of the vegetable oil to saturated product was obtained after 3 h. 3.3. Leaching and Catalyst Reusability. Leaching of Pd from the catalyst was in each case tested by ICP-OES in the reaction products. Solid Pd catalysts are notorious leachers12, and we have recently investigated the leaching issue in heterogeneous Pd catalysis.13 Here, catalysis was truly heterogeneous, as shown by lack of reactivity of the reaction filtrate obtained after 20 min of oleic acid conversion under the optimized reaction conditions of Table 1. In general, the last columns in Tables 1, 3, and 4 show that varying the reaction conditions in the hydrogenation of oleic acid (Table 1) or the FA (Table 3) or vegetable oil (Table 4) substrates resulted in values of leached Pd in the isolated crude product that are
